Normal fetal growth is a critical aspect underlying human development and has implications across the lifespan. Currently, few standards exists for the clinical assessment of fetal growth, particularly race/ethnic specific. In response, the Division of Epidemiology, Statistics & Prevention Research designed the NICHD Fetal Growth Study whose aims are threefold: (1) establish a national standard for normal fetal growth (velocity) and size for gestational age among low risk infants; (2) create an individualized standard for optimal fetal growth; and (3) improve the accuracy of clinical fetal weight estimation. This study will recruit 2,400 healthy, non-obese, low risk pregnant women from 12 participating clinical centers. Approximately 600 women each will come from the following self-identified race/ethnicity background: African American, Asian, Caucasian, and Hispanics. Women will be recruited in the first trimester and followed up through pregnancy. In addition, 600 obese women will also be recruited. Women will be randomized to receive gestation-specific 2D and 3D ultrasounds. Data will be collected from women at each clinical visit to ascertain diet, physical activity and health information and also at delivery. Anthropometric assessment of the pregnant women and baby also are conducted to assess body composition. An February 2012, a follow on protocol was implemented to include 350 dichorionic twins. The aim of this study is to establish a contemporary growth trajectory for twins, and to compare it with that of singletons. This information is critical fr the clinicians, given the epidemic of twin births in contemporary birth cohorts.